In the state of the art, in order to detect the mutual or reciprocal position between a cylinder and a respective piston slidable in it, the piston rod rigid with the piston, e. g. made of steel, is provided with one or more reference areas, which can be detected by suitable detecting means provided on the respective cylinder. More particularly, each reference area is typically obtained on the surface of the piston rod, e. g. through a laser processing along a larger or lesser extension of an arc of circumference of the circular cross-section thereof, and extends along the piston rod axis through a predetermined length usually ranging from 1 mm to 7 mm according to the operator's requirements.
The detecting means of a reference area are usually provided at one of the two ends of the cylinder, received in a receiving seat formed therein and facing the piston rod of the cylinder-piston unit. Such detecting means are designed to detect, at a detection area or region thereof, the presence or absence of the reference area on the piston rod, once the reference area lies at such detection zone during the piston stroke within the cylinder.
The detecting means usually used for detecting a reference area on the piston rod are based on the difference in the reflected light radiation incident on the piston rod either inside or outside the reference area. To this end, they usually include a emitting means light radiation incident, in use, onto the piston rod surface at the detection area, and a respective receiving means arranged to receive such radiation reflected by the piston rod surface. The light radiation is almost totally reflected by the piston rod surface not bearing a reference area, whereas the radiation is absorbed to a greater extent by the surface bearing the reference area. Accordingly, the electrical signal to be processed at the receiving means output, which is typically of an analogic type, will have a lower amplitude at a reference area with respect to that detectable at a piston rod portion not bearing such area. The output signal generated by the receiving means is subsequently processed by a suitable data processing unit. At the output of such data processing unit, once the reference area has been detected on the piston rod, a suitable electrical detection signal is generated, typically having a step-shape, which will be used for subsequent processing depending upon the use of the cylinder-piston unit.
For example, should the cylinder-piston unit be mounted in a known way on the axle of a car provided with steering wheels and the detection of the reference area by the detecting means correspond to the laying of each wheel on a plane orthogonal to the longitudinal axis of the axle, the detection signal can be sent to the control unit of the car for blocking the wheel movement on such orthogonal plane.
The cylinder-piston units of the state of the art suffer from a number of drawbacks.
First of all, the detecting means in time detect the presence or absence of the reference area on the piston rod in an increasingly less accurate way, because it becomes deteriorated due to prolonged use of the cylinder-piston unit. The often extreme working conditions, e. g. very low or very high temperatures, also affect the operation of the detecting means and the other components of the cylinder-piston unit, which results in an unreliable spotting of, or failure to spot the reference area at the detection area by the detecting means.
Another limitation of the state of the art cylinder-piston units is that they do not meet the requirements of the operators in the art who experience the need of adjusting the trend of the detection signal (in jargon referred to as “field of action”) in a simple and economic manner depending upon the applications. As is known, the trend of such an electrical detection signal also depends on the size of the reference area provided on the piston rodpiston rod.
In some applications, it may happen that the electrical detection signal at the output of the data processing unit is used for turning on a warning light, and that as a response to the warning light being turned on an operator is expected to implement specific operations. In this case, it is preferable that the warning light is kept on or off for sufficiently long time intervals, so that a clearly interpretable signal is sent to the operator. However, it often happens that the detection signal is swinging owing to vibrations the cylinder-piston unit is experiencing in some specific applications, or because the time interval during which the reference area remains at the detecting means is short. For these reasons, the warning light goes on being switched on and off, thereby sending a signal which is difficult to be interpreted by the operator.
In order to compensate the undesired effect mentioned above, it is preferable that the width of the reference area on the piston rod is large, so that the output signal correlated to it from the detecting means, either high or low, is stable for a longer time, the working conditions being the same, with respect to the case in which the width of the reference area is smaller along the piston rod.
In other circumstances, for example when the output detection signal from the data processing unit is sent to an automated actuator assembly, it is preferable that the automated actuator assembly is promptly energized upon the detection signal being generated. In this case, it would be preferable that the width of the detection area along the piston rod is reduced, or in other words the “field of action” is more limited or narrow. In the case referred to above, where for example the cylinder-piston unit is mounted in a known manner on the axle of a car provided with steering wheels and the detection of the reference area by the detecting means corresponds to the laying of each wheel in a plane orthogonal to the longitudinal axis of the axle, the narrower the width of the reference area along the piston rod, the more accurate and reliable the detection of the position of each wheel in such a plane. The automated actuator assembly will be energized in order to limit the rotation of each wheel in such plane, as soon as the detection signal is generated.
As mentioned above, in order to adjust the shape of the output detection signal of the data processing unit, the operator, up to now, has only had the possibility of having cylinder-piston units provided with reference areas of different widths along the piston rod axis, and mounting each time the one provided with adequate dimensions in order to satisfy his needs.
It is clear that this procedure is economically inconvenient for the operator, as well as complex to be implemented, since it requires, each time, the selection, among the available cylinder-piston units, of the one which is more suitable, given the circumstances, and the use of one or another.